Download 71 A characterising of the ore minerals due to mineralogical

A characterising of the ore minerals due to mineralogical, chemical and textural
properties in Malmberget.
Cecilia Lund 1, Olof Martinsson 1
Luleå University of Technology1,
971 87 Luleå, Sweden
(e-mail: [email protected])
Abstract
This study was an attempt to find a way of characterising an iron ore body both
mineralogical and textural in a quantitative manner by using analytical methods like
optical microscopy, microprobe (EMPA) and an automatic SEM based system, Particle
Texture Analysis (PTA). The source of this study is an iron ore body, called Fabian,
located in Malmberget, Sweden. Two types of ores were identified and analysed in this
study named “orebreccia” and “ore”. The Particle Texture Analysis was made on two
fractions of crushed ore. The mineralogy was evaluated and characterized as mineral
liberation and mineral association. Magnetite has a simple outline and straight grain
boundaries and the gangue minerals have a finer particle size with a more complicated
texture. The liberation of magnetite in “ore” and “ore breccia” is high. The ore quality
for both “ore” and “ore breccia” does have similarities in a process technique
perspective.
Introduction
LKAB is today one of the world’s leading producers of upgraded iron ore products. The
customers expect good qualities of the products which demand a detailed knowledge of
the processes from mine to customer. To achieve this it is important to get a traceability
of ore feed and control over the production process but it will also demand a good
knowledge of the raw material.
Concept of process mineralogy
Through studying the process mineralogy of iron ore it is possible to understand the
behaviour of the ore feed during the processing plant and also later on the sintering and
pelletising processes. It is needed to know what kind of minerals and textures the ore is
consisting of due to the fact that the liberation characteristics are intimately related to the
mineralogical texture (Lorenzen and van Deventer 1994).
Optical microscopy has traditionally been the instrument used for the identification and
quantification for both mineralogical and texturally properties, which is a time
consuming process (Petruk 2000). A number of different techniques according to image
analyses system based on SEM techniques have been developed during the three last
decades for a more rapid quantitative description of mineralogy and particle textures
(Jones and Gravilovic 1970; Gottlieb, Wilkie et al. 2000; Petruk 2000; Gu 2003).
In this study which is a part of a larger thesis, a determination of the mineral properties
of one specific iron ore body from LKAB Malmberget was carried out. This was an
attempt to find a way of characterise the mineralogy and textures of the iron ore body.
Three methods were used; optical microscopy, microprobe and automatic SEM based
system, Particle Texture Analysis (PTA) (Moen 2006).
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The Fabian ore body is major Fe resource at Malmberget since the last drill program has
shown an expanded ore body. The ore body plunge in ~ 80° to a depth of at least 1250
meters. The length and width is approximately 700 to 900 m and 50 to 150 m
respectively. The ore quality is also better then average for the Malmberget deposit, due
to the higher Fe-content (fig. 2).
Geological setting
Regional geology:
The geology of the ore province of northern Norrbotten is compound of bedrock
sequences with different ages. The oldest rocks are an Archaean granitoid-gneiss
basement with dated ages of 2.8Ga. Unconformably overlaying sequences are
supracrustal successions of Palaeoproterozoic age 2.5-2.0 Ga, followed by Svecofennian
volcanic and sedimentary units, dated c. 1.9 Ga. Forty apatite-iron deposits are known
from the northern Norrbotten ore province and they are hosted and probably genetically
related to the volcanic rocks in the Svecofennian succession (Martinsson 2004).
The two economically most important deposits are mined by LKAB at Kiruna and
Malmberget. These two apatite-iron ore deposits have a similar origin and were formed
by magmatic-hydrothermal process at 1.89-1.88 Ga (Bergman, Kübler et al. 2001).
However, there are major discrepancies in character between them, due to later
overprinting by metamorphoses, deformation and granitic intrusions, which are stronger
for the Malmberget ore.
Deposit geology:
More than 20 different tabular to stock shaped ore bodies are known in the Malmberget
ore field, spread over an area of 2.5 x 5km. There exist only one generally description of
Malmberget deposit written by (Geijer 1930).
The Malmberget deposit were probably from the beginning a continuous ore lens which
were exposed for at least two phases of folding and metamorphism. These events torn
the ore lenses apart by strong ductile deformation and today they occupy a large-scale
fold structure were the individual ore bodies stretch parallel to the fold axis, which
plunge 40º-50º towards SSW (Bergman, Kübler et al. 2001) (fig. 1).
Fig. 1 A simplified geological map of Malmberget. From Bergman et al. (2001).
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The iron ore minerals are both magnetite (Fe3O4) and hematite (Fe2O3) but the
magnetite is more common of the two. Hematite forms several separate ore bodies and
portions of others (Geijer 1930). The main gangue minerals are apatite, amphibole,
pyroxene, feldspars, quartz and biotite. Among the accessory minerals are pyrite,
chalcopyrite, titanite, zircons and calcite most common. Every ore body is characterised
by there own mineral, chemical and textural properties.
Due to the strong metamorphic recrystallization of the area, the minerals are
recrystallised, coarse grained, and elongated in the direction of the lineation of the rocks.
Fig. 2. A schematic picture of the Fabian ore body.
Analytical methods
The analytical methods which were used are optical microscopy, microprobe (EMPA)
and Scanning electron microscopy (SEM-PTA).
Sample preparation
For optical microscopy and microprobe, samples of intact ore were used and for the
SEM-(PTA) method, the samples were crushed and sieved into sized fractions.
Drill core samples from various parts of the ore were taken to characterise different
textures. Polished thin sections were made of both intact ore and two particle sizes from
sieved fractions of crushed ore (tab. 1).
The sized samples were of a larger volume since the sample should be representative for a
specific ore type and not only for a specific texture.
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Ore types:
Two types of ore were identified and used in this study, called “ore breccia” and “ore”.
The “ore breccia” is largely consisting of magnetite but do have gangue minerals like
quartz, amphibole, pyroxene, apatite and feldspars in different proportion occurring as
breccia infill in the wall rocks. The “ore” is more massive magnetite that contains gangue
minerals like amphiboles and apatite in fewer amounts. The “ore breccia” is bordering
the massive ore, but occurs also partly as inclusions in the massive ore (fig. 3).
Table 1. The samples which is used in the study.
Sample
Intact ore
Mbgt 6500 100.22-100.30 (AP1)
x
x
x
x
x
x
Mbgt 6500 111.00-111.13 (AP3)
Mbgt 6500 112.57-112.63 (AP4)
Mbgt 6500 374.23-374.35 (B32585)
Mbgt 6500 383.21-383.27 (A32584)
Mbgt 6500 435.56-435.64 (C32586)
Mbgt 6500 128.7-129.8 (Ore breccia)
Mbgt 6500 424.53-426.92 (Ore)
150 µm
75 µm
x
x
x
x
At the mineral processing laboratory at LTU the samples were crushed in a Retsch jaw
crush, +3 mm, split by a Jones splitter and sieved with a Ro-Tap shaker at the fraction
150 μm and 75 μm.
Optical microscopy
Polished thin section were optical examined in transmitted and reflected light on a
standard petrographical microscopy (Nikon Eclipse E600).
Fig. 3. Two ore types. The red spots are defined textures and parageneses.
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To characterise all the different mineral associations, textures and parageneses, a mineral
identification were made of both the silicates and oxides. 23 different spots were analysed
on the major minerals in both ore types, “ore breccia” and “ore” (Fig. 3)
Microprobe
Mineral analyses were performed on a JOEL JXA-8500F electron microprobe at
NTNU, Trondheim, Norway. For the microprobe analyses an accelerating voltage at
15.0 kV, a probe current at 95 μA and a < 1μm beam diameter were used.
Totally 166 analyses were made, representing both silicates and oxides on the major
minerals in 23 spots to cover all different textural and mineral assemblage variations,
observed in the samples.
Beside this mineral identification, it also verified that the used sized fraction samples were
representative.
Scanning electron microscopy
The particle analyses were performed on a Hitachi S-4300SE scanning electron
microscopy equipped with Oxford Inca Feature software NTNU, Trondheim, Norway.
For the particle analyses an accelerating voltage at 20.0 kV and a probe current at
~0.5nA were used.
Particle texture analysis PTA
By using Back Scattered Electron (BSE), images are analysed by means of grey levels and
every grain of interest was analysed by X-rays. Every analyzed grain size fraction is
imported to the PTA software. Images analysis is performed offline to process and
evaluate if grains occur liberated or in composite particles.
Standard queries can be performed from the output results in a new database such as the
mineral liberation of any mineral, mineral association of any mineral and miniature
images of particles of a certain texture category (Moen 2006).
To reduce the unclassified group of minerals an extensive identification of minerals and
phases for classification should be done.
Results
Mineralogy
The textures of the”ore breccia” is characterised by magnetite grains with a simple
outline and straight grain boundaries, either as single grains or as a particles in a matrix of
quartz and feldspar. This matrix has a myrmecitic texture and the grains shows anhedral
granular outline with complicated grain boundaries (fig 4).
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Fig. 4. Photographs of the mineralogy and textures from “ore breccia”, (upper photos) and
“ore” (lowest photo). Mag, magnetite, ap, apatite, qtz, quarts, amp, amphibole, bt, biotite.
In the “ore” the texture of magnetite is dominated by grains having simple outlines with
straight grain boundaries. The grains are of different size with coarser grains often
elongated in the direction of lineation occurring in a finer grained matrix. Gangue
minerals like apatite, amphiboles and biotite are elongated in a linear direction (tab. 2).
Table 2. Sample descriptions of the mineralogy and texture.
Mag
Hem
Ap
Pl
Mc
Qtz
Amp
Mbgt 6500
100.22-100.30
x
x
x
x
x
x
Mbgt 6500
111.00-111.13
x
x
x
x
x
x
Mbgt 6500
112.57-112.63
x
x
x
x
x
x
Mbgt 6500
374.23-374.35
x
x
x
x
x
Cal
x
Bt
x
x
Py
Texture
Single simple outlined mag grain
or aggregate in a myrmecitic
matrix of fsp-qtz.
Single simple outlined mag grain
or aggregate in a myrmecitic
matrix of fsp-qtz.
Single simple outlined mag grain
or aggregate in a myrmecitic
matrix of fsp-qtz.
Massive mag. fine grained at the
contact to amphibole.
Massive mag, homogenous, fine
grained matrix, elongated coarse
grains.
Massive mag, elongated coarse
grains. Ap and amp grains
Mbgt 6500
x
x
x
435.56-435.64 x
elongated in the direction
lineation.
Mag, magnetite, hem, hematite, ap, apatite, pl, plagioclase, mc, microcline, qtz, quarts, amp, amphibole, cal, calcite, bt, biotite, py,
pyrite.
Mbgt 6500
383.21-383.27
x
x
x
x
Particle texture analysis (PTA)
The modal mineralogy for the “ore breccia” samples shows slightly different results when
comparing the fractions 150 µm and 75 µm. Magnetite decreases with 8.2 % in the 75 µm
fraction. The classes Mg(Ca)- silicate and quartz/feldspar are also decreasing in the smaller
fraction. Instead the albite and actinolite class increases, about 14 %, however, actinolite is
not represented at all in 150 µm (fig. 5), (tab. 3).
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Fig. 5. The modal mineralogy of the different mineral classes divided in particle size fractions.
The two particle fractions in the “ore” samples do have the same mineral classes
represented but in different volume proportion. The magnetite class decreases 5 % from
the 150 µm to the 75 µm fraction. The amphibole - pyroxene minerals (actinolite) and
(Mg(Ca)-silicate) constitute together 5 % in the 150 µm fraction and increases to 7% in
the 75 µm fraction which also become the second largest group (fig. 5). In both ore
types there is an unclassified class that include particles that could not be determinate
because of an incomplete x-ray analysis.
Table 3. Mineral classes and number of grains in each fractions.
Ore
breccia
Mineral
Apatite
Magnetite/hematite
Quartz
Calcite
Biotite
Pyrite
Titanite
Ortoklas/microcline
Actinolite/Augit
Dolomite
Albite
Fe-Ti-oxide
Quartz/Feldspar
Mg(Ca)-silicate
Calcite/apatite
Zircon
Unclassified
Total
Number
3
10691
592
46
1
3
145
614
0
0
157
133
911
884
7
1700
1385
17272
150 µm
w%
0,01
64,75
2,48
0,08
0
0
0,08
2,26
0
0
3,32
0,24
9,78
11,04
0
5,98
2,35
100
Ore
breccia
Number
8
4213
173
73
48
0
189
827
943
0
1095
79
70
369
3
6
528
8624
75 µm
w%
0,04
56,52
2,43
0,32
0,41
0
0,34
3,92
13,51
0
17,76
0,24
0,57
3,88
0
0,07
0,71
100
Ore
Number
161
7293
60
1
14
18
131
8
302
0
65
203
8
397
14
0
1420
10095
150 µm
w%
1,62
91,13
0,84
0
0,02
0,15
0,13
0
3,3
0
0,14
0,55
0
2,1
0
0
0,79
100
Ore
Number
170
10214
77
26
19
108
218
30
963
1
61
433
14
1280
24
2
1700
15340
75 µm
w%
1,71
87,31
1,17
0,05
0,03
0,11
0,18
0,02
4,68
0
0,6
0,91
0,03
3,18
0,01
0
1,39
100
At the liberation analysis, classes of similar minerals were fit into broader groups.
Amphiboles*: actinolite + Mg(Ca)-silicate, Feldspar*: Qtz/Fsp + orthoclase/microcline + albite
and the remaining classes are Magnetite och Quartz.
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Fig. 6. Liberation of minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm).
In fig. 6, yellow colour means that 100 % of the magnetite is classified as an apparently
liberated grain and orange ditto means that more than 95 % is liberated. The degree of
liberation of magnetite is high and quite similar for the both ore types ~ 85 %. It shows a
slight increase in the finer fraction. Magnetite with more than 95 % liberated grains will
almost be the remaining part in every fraction.
The liberation for the different silicate groups is not as high as magnetite. Quartz and
Amphibole* classes are more liberated in the finer 75 μm fraction (fig. 6).
Fig. 7. Magnetite associated with other minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm).
The mineral classes which is associated to magnetite is more diverse in the “ore breccia”.
Amphibole* is associated to magnetite in the “ore” (fig. 7 & 8).
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Fig. 8. Amphibole* associated with other minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm).
Discussion
As pointed out in the introduction this study was an attempt to find a way of characterise
the mineralogy and textures of an iron ore body. The different ore types were identified
during the geological mapping and interpreted in the optical microscopy. Both these ore
type were quantified by the mineral content, mineral liberation and mineral association
by the particle texture analysis (PTA).
The modal mineralogy for this study shows a decreasing magnetite proportion at the
finer fraction. This is probably due to the primary grain size in the samples are larger for
magnetite compared to the gangue minerals.
The liberation of the silicate groups indicates that the primary grain size is finer. The
“ore breccia” has more and larger mineral classes like quartz, feldspar and amphibole
fractionated at 75µm particle size. The texture for the silicate matrix is more complicated
in comparison to the coarse grained magnetite which has simple grain boundaries. The
interpretation is also verified in the optical microscopy
This study will be extended to include more samples of the same ore type but also other
ore types found in the Fabian ore body. Other ore bodies will also be included to verify
that the results could be applicated in a general way for the Malmberget deposit.
Conclusions
The primary grain size for magnetite is coarser and has a higher proportion at the coarser
fraction 150 µm.
Differences in the modal mineralogy of magnetite and silicates in different particle sizes
of the “ore breccia” are close connected to the texture.
The liberation of magnetite in “ore” and “ore breccia” is high and being quite similar.
The ore quality for both “ore” and “ore breccia” does have similarities in a process
technique prospective.
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Acknowledgement
Thanks for the financial support by Hjalmar Lundbohm Research Centre (HLRC). I am
grateful to Prof. Terje Malvik and Dr. Kari Moen, NTNU for all help during the
particle texture analysis and their great knowledge of process mineralogy.
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